1. Field of the Invention
The present invention relates to a method and an apparatus for injection molding a metal material, and more particularly to a method and an apparatus for injection molding a metal material, by which nonferrous metal having a low melting point, such as zinc, magnesium, or alloy thereof, is completely melted to allow injection molding in a liquid phase state.
2. Detailed Description of the Prior Art
Attempts have been made to completely melt nonferrous metal having a low melting point so as to allow injection molding in a liquid phase state. Like in the case of injection molding of a plastic material, the molding method thereof adopts a heating cylinder having inside an injecting screw, which is allowed to rotate and move along the axial direction. A granular metal material supplied from the rear portion of the heating cylinder is heated and melted completely while being transferred toward the head of the heating cylinder by means of rotation of the screw, and after a quantity of the metal material in the liquid phase state is metered in the front chamber of the heating cylinder, the metal material is injected to fill a mold through the nozzle attached to the tip of the heating cylinder by moving the screw forward.
A problem occurring in case of adopting the foregoing injection molding for the metal material is that the material is neither transferred readily nor metered in a stable manner by means of rotation of the screw.
A molten plastic material has a high viscosity, and transfer of the molten plastic material by means of rotation of the screw is allowed mainly because a friction coefficient at the interface of the molten plastic material and the screw is smaller than a friction coefficient at the interface of the molten plastic material and the inner wall of the heating cylinder, and therefore, a difference in friction coefficient is produced between the two interfaces.
In contrast, the metal material, when melted completely to the liquid phase state, has such a low viscosity compared with the plastic material that a difference in friction coefficient is hardly produced between the above two interfaces. Hence, a transfer force such as the one produced with the molten plastic material by means of rotation of the screw is not readily produced.
However, a transfer force is produced with the metal material when it is in a solid state and in a high viscous region where the metal material is in a semi-molten state during the melting process. Thus, the metal material can be transferred by means of rotation of the screw up to that region. Nevertheless, as the metal material is further melted, the viscosity drops with an increasing ratio of the liquid phase, and the transfer force produced by the screw grooves between the adjacent screw flights decreases, thereby making it difficult to supply the molten metal material in a stable manner to the front chamber of the heating cylinder by means of rotation of the screw.
Because the molten plastic material has a high viscosity, it is stored in the front chamber of the heating cylinder by means of rotation of the screw, while at the same time, a material pressure that pushes the screw backward is produced as a reaction. By controlling the screw retraction caused by the material pressure, a constant quantity of the molten material can be metered each time.
However, the metal material in the low-viscous liquid phase state cannot produce a pressure high enough to push the screw backward. Thus, the screw retraction by the material pressure hardly occurs, and if the metal material is stored in the front chamber by means of rotation of the screw alone, a quantity thereof undesirably varies, thereby making it impossible to meter a constant quantity each time.
In addition, the metal material has a far larger specific gravity compared with the plastic material, and has a low viscosity and fluidity in the liquid phase state. For this reason, when allowed to stand by stopping rotation of the screw, the metal material in the liquid phase state in the heating cylinder placed in a horizontal position leaks into the semi-molten region in the rear portion through a clearance formed between the screw flights and heating cylinder. Consequently, the metal material accumulated in the front chamber causes a back flow onto the periphery of the head portion of the screw through the opened ring valve, and the quantity thereof is undesirably reduced.
The liquid level in the front chamber is lowered with the decreasing accumulation quantity. For this reason, a gaseous phase (space) that makes the metering unstable is generated at the upper portion of the front chamber. In addition, the leaked metal material in the liquid phase state increases its viscosity in the semi-molten region as its temperature drops, or turns into solid depending on the heating condition in the semi-molten region, thereby forming weirs in the screw grooves. This poses a problem that the granular material supplied from the supply port provided behind the weirs cannot be transferred readily by means of rotation of the screw.
The present invention is devised to solve the above problems raised with injection molding of a metal material in the liquid phase state, and therefore, has an object to provide a novel method and apparatus for injection molding a metal material, by which the metal material in the liquid phase state can be transferred, metered, and deaerated smoothly at all times by adopting means of placing an injection apparatus in an inclined position.
In order to achieve the above and other objects, the present invention provides an injection molding apparatus of a metal material composed of an injection apparatus having a heating cylinder provided with a nozzle at a tip thereof and a supply port at a rear portion thereof and having inside a screw, which is allowed to rotate and move along an axial direction, and a clamping apparatus provided to oppose the injection apparatus and equipped with a mold having inside a sprue bush, in which the injection apparatus and clamping apparatus are placed on an apparatus platform in an inclined position at a same angle with the mold in a lower end, so that the metal material in a liquid phase state in the heating cylinder flows down into a front chamber of the heating cylinder due to self-weight, and that the nozzle and the sprue bush inside the mold are positioned on a same straight line, thereby maintaining nozzle touch without bending the nozzle.
The screw may include an injecting plunger at the tip thereof. The plunger has substantially a same diameter as a diameter of the front chamber formed in the heating cylinder at a top end portion by reducing a diameter thereof so as to be allowed to fit into the front chamber by moving forward and backward while securing a sliding clearance such that hardly causes a back flow of a liquid phase material accumulated in the front chamber.
Further, the present invention provides an injection molding method of a metal material including: placing both (1) an injection apparatus having a heating cylinder provided with a nozzle at a tip thereof and a supply port at a rear portion thereof and having inside a screw, which is allowed to rotate and move along an axial direction, and (2) a mold provided to oppose the injection apparatus and having inside a sprue bush in an inclined position at a same angle with the mold in a lower end, so that a metal material in a liquid phase state in the heating cylinder flows down into a front chamber of the heating cylinder due to self-weight; accumulating and metering the metal material in the liquid phase state in the front chamber of the heating cylinder by means of flowing due to an inclination and rotation of the screw; and injecting the metal material to fill the mold.
In the present invention, it is preferable to provide a sensor for counting the number of revolutions of the screw, so that the number of revolutions of the screw is controlled to stay at a set number of revolutions by means of the sensor.
As has been discussed, according to the present invention, by placing both the injection apparatus and mold in an inclined position at the same angle, the metal material in the liquid phase state can be accumulated and metered in the front chamber by means of flowing due to an inclination and rotation of the screw. Therefore, even when the metal material is in the low-viscous liquid phase state, the metal material can be transferred readily and smoothly by the screw. In addition, the liquid surface faces a gaseous phase produced at the interface between a semi-molten material and the liquid phase material in a horizontal position, and the semi-molten material is positioned upper than the liquid surface. For this reason, even when allowed to stand by stopping the rotation of the screw, the liquid phase material does not leak into the semi-molten material side, thereby preventing unwanted variance in an accumulation quantity in the front chamber. Further, the metal material in the liquid phase state can be stored primarily and an accumulation quantity in the front chamber can be compensated by means of rotation of the screw. Consequently, a product with a stable molding state can be obtained from a metal material even by means of injection molding of a metal material in the liquid phase state.
Also, because the injection apparatus and mold are placed on the apparatus platform in an inclined position at the same angle with the mold at the lower end so that the nozzle and the sprue bush inside the mold are positioned on the same straight line to maintain the nozzle touch without bending the nozzle, the moving direction of the nozzle and the opening/closing direction of the mold have a common axis. Consequently, the tip end surface of the nozzle and the nozzle touch surface of the sprue bush can be formed in the typical manner, while at the same time, leaking of the material caused often by deficient nozzle touch can be prevented.